Australia's Science Forum

I have 2 recipes for Agar .. one uses Peptone and one Pepsin.. which is best and what is the difference between them..
Also my peptone has gone hard anfd a dark yellow colour - has it gone 'off'?

I can tell you the difference between them, but not which you should use. Our recipe uses peptones which from their definitions seems the logical choice.

Pepsin: a digestive enzyme produced in the stomach that converts proteins into peptones; it only acts in a acid medium.

Peptones: Organic substances produced by hydrolysis of proteins by the action of pepsin in the stomach. They are soluable in water and are absorbed by the body.

Wendy

Our peptone is a buff yellow, but reasonable hard, but has always been like it. It smells like nutrient agar, so I suspect that the correct ingredient is peptone.

I have always used nutrient agar to make agar plates for over 15 years. They always work and are dead easy to make, and wouldn't bother making "a brew". So if you can get hold of it, it is worth it.

Peptone is added to agar as a source of protein. When we ran out of peptone I started to use gelatine which we have in abundance. Gelatine is also a protein and much easier to source - your local friendly supermarket. It works equally well as peptone. Our peptone used to stink after a short time, another reason for not using it.

Cheers,

Robbie

I might have to start using gelatine as well, it would be a lot cheaper and easier to get hold of and the Peptone I have is extremely hard and it is annoying when I have to try and use it.

What amount of Gelatine do you use, same as for peptone??? I think I will give this a go.

Yes, I use the same amount of gelatine that I used for peptone.
Haven't had any problems using it.

Robbie

When I buy a new bottle of peptone I weigh 5g lots of the powder, store it in small (40x60mm) snaplock bags, store the bags in a sealed plastic container and keep the container in the freezer. It doesn't stop the peptone from darkening or going hard, but it's alot easier to peel the plastic bag off a lump of peptone than trying to gouge it out of a bottle. It dissolves easily, no waste and still works after 5 years or more.
Cheers,
Fran

Well done Fran, just another great idea.

Thank you